It is common to use metal pipe as a conduit for the conveyance of particulate matter from one point to another. An example is the transportation of finely crushed coal from a pulverizer to a combustion chamber or “firebox” where the pulverized coal is ignited and the heat used to make steam which powers a turbine. Metal conduits are also used to transport mineral ore and many other materials in extraction and refining operations.
In some conduit arrangements, it is essential to be able to regulate the flow of material through the conduit; for example, it is often desirable to balance the flow of pulverized coal flowing through multiple, parallel conduits to supply points around a combustion chamber thereby to control the shape and location of the fireball within a the combustion chamber.
Various types of valves or variable orifices have been designed for this purpose. Some such devices are gate type valves in which a blade structure is set up to slide variably across the interior cross-section of the conduit. There are numerous disadvantages associated with gate valves of this type. Other orifices use variable angle blades.